3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
An LTE system based on an orthogonal frequency division multiplexing (OFDM) technique has a possibility that an interference to another user occurs in a process of performing communication between a user equipment (UE) and a base station (BS). In order to minimize the interference, it is very important for the BS to manage uplink transmission timing of the UE. Therefore, the BS properly regulates the uplink transmission timing of the UE according to a situation of the UE, and such a regulation is called management of uplink time alignment.
Since the UE does not always exist in a fixed location, the transmission timing of the UE varies depending on a speed and location of the moving UE. By considering this, the UE assumes that the uplink time alignment is valid only during a specific time period even if the uplink time alignment is set. A timer used for this is a time alignment timer (TAT).
The TAT is used to control how long the UE maintains the uplink time alignment.
The UE uses various applications. One application may need to always maintain the uplink alignment, whereas another application may no longer need to maintain the uplink alignment due to temporary uplink transmission.
Although the application used by the UE has various features, whether to maintain an uplink synchronized state of the UE is determined entirely by the BS in the LTE system.
There is a need to determine whether to maintain the uplink time alignment according to the situation of the UE.